A ceramic sintered body has properties of light weight, high hardness, high wear resistance, high anti-corrosion resistance and low thermal expansion coefficient, so that the ceramic sintered body has been widely used as a member for constituting precious devices. In particular, from the viewpoint of excellence in high hardness and the wear resistance, the ceramic sintered body has been preferably utilized as the wear resistant member constituting a bearing. Among various ceramic sintered bodies, a silicon nitride (Si3N4) sintered body has a high hardness and an excellent wear resistance, so that the silicon nitride sintered body has been preferably used as a member for constituting the bearings or the like.
Regarding this silicon nitride sintered body, for the purpose of improving a reliability as the wear resistant member for constituting the bearings or the like, further improvements in properties have been advanced in these days. For example, there has been proposed a method of manufacturing the silicon nitride sintered body, comprising the steps of:
preparing a material powder mixture by adding yttrium oxide, spinel, aluminum oxide and/or aluminum nitride to a silicon nitride material powder at predetermined molar ratio of specified metal elements and predetermined content ratio;
preparing a compact body (molded body) formed from the material powder mixture;
sintering the compact body at a temperature of 1400 to 1500° C.; further sintering the compact body at a temperature of 1500 to 1650° C. thereby to obtain a sintered body having a relative density of about 98%; and                conducting a secondary sintering (main sintering) operation to the sintered body at a temperature of 1400 to 1650° C. in a nitrogen gas atmosphere of 10 atm or higher thereby to increase the relative density to be about 99%, so that there can be manufactured a silicon nitride sintered body excellent in strength and having less scattering in strength properties (for example, refer to Patent Document 1).        
Further, as the silicon nitride raw material powder used for manufacturing this type of the silicon nitride sintered body, it is generally known that a high-purity material powder is preferably used. for example, a high purity material powder synthesized through an imido thermal decomposition method has been suitably used.
However, this high-purity material powder is costly expensive, and mechanical strength and fracture toughness values of thus manufactured silicon nitride sintered body are tend to become excessively large, so that there is posed a problem such that a workability of the resultant sintered body is not sufficient.
In order to solve this problem, there has been reviewed a method of manufacturing the silicon nitride sintered body by using a cheap silicon nitride raw material powder manufactured through a direct nitriding method in which a metal Si is directly nitrided. The silicon nitride raw material powder manufactured through the direct nitriding method has relatively large Fe and Ca contents.
However, it is known that when the contents of the rare earth element, aluminum component and silicon carbide or the like are controlled to be within predetermined ranges, so that there can be obtained a sintered body having a mechanical strength, a wear resistance and a rolling life property that are equal to or more than those of conventional sintered bodies, and the resultant sintered body is also excellent in workability (for example, refer to Patent Document 2).
As described above, when the compact (molded body) formed of the material powder mixture containing the silicon nitride raw material powder is subjected to a primary sintering, a sintered body having a relative density of about 98% is obtained. Thereafter, the resultant sintered body is further subjected to a secondary sintering operation in a nitrogen gas atmosphere having a pressure of 10 atm or higher so as to obtain a sintered body having a relative density of over 98%; preferably 99% or higher, thereby to manufacture a silicon nitride sintered body excellent in strength and having a less scattering in strength properties.
However, in order to obtain the relative density of about 98% at the primary sintering operation, it is necessary to strictly control the manufacturing process, so that there is posed a problem such that a production cost required for manufacturing the silicon nitride sintered body is disadvantageously increased.
As described above, the cost of the silicon nitride material powder manufactured through the direct nitriding method is relatively low. Further, when the amounts of rare earth element, aluminum component and silicon carbide or the like contained in the silicon nitride material powder are controlled to be within predetermined ranges, there can be manufactured a silicon nitride sintered body excellent in mechanical strength, wear resistance and rolling life property or the like, especially excellent in workability.